1. Field of the Invention
The present invention relates in general to a speed sensing apparatus for detecting or determining the ground speed of a motor vehicle relative to the ground or road surface, by utilizing a Doppler effect or Doppler shift, and more particularly to improvements in such a Doppler-effect vehicle ground speed detecting apparatus which has means for adjusting the frequency of a wave generated by a transmitter of the apparatus.
2. Discussion of the Prior Art
A common Doppler-effect vehicle ground speed detecting apparatus includes (a) a transmitter which generates and transmits a wave towards the ground surface, (b) a receiver which receives a portion of the wave reflected by the ground surface, and (c) output means for producing an output indicative of the vehicle ground speed, on the basis of the frequency of the wave as transmitted from the transmitter, and the frequency of the wave as received by the receiver, and according to the principle of the well known "Doppler effect" or "Doppler shift".
Generally, a Doppler-effect ground speed detecting apparatus has a tendency that the sensitivity of the receiver to the reflected wave has some dependence upon the frequency of the wave received. Usually, the sensitivity of the receiver has a peak at a given frequency of the received wave, and is represented by a frequency-sensitivity curve which is convex upwards. On the other hand, the frequency of the wave received by the receiver (hereinafter referred to as "receiving frequency") changes with a change in the ground speed of the vehicle, provided that the frequency of the wave generated by the transmitter (hereinafter referred to as "transmitting frequency") is kept unchanged. Accordingly, the sensitivity of the receiver changes as the actual ground speed of the vehicle changes, if the transmitting frequency is constant. In other words, it is difficult to hold the sensitivity of the receiver at a high level within an optimum range, irrespective of a change in the vehicle ground speed, if the transmitting frequency of the transmitter is held constant.
To obviate the above drawback, an improved ultrasonic type Doppler-effect ground speed detecting apparatus is proposed as disclosed in U.S. Pat. Nos. 5,097,453 and 5,148,409 which is equipped with frequency changing means for adjusting or changing the transmitting frequency, on the basis of the receiving frequency, so as to control the receiving frequency of the receiver, so that the sensitivity of the receiver to the received wave is always held in an optimum range.
For assuring high accuracy of detection of the vehicle ground speed by such a Doppler-effect speed detecting apparatus, it is important that the wave transmitted from the transmitter towards the ground or road surface be irregularly reflected by the ground surface in the presence of an adequate degree of roughness or waviness of the surface, so that a sufficiently large amount of the wave generated by the transmitter is received by the receiver. However, the ground surface usually has local smooth zones such as plashes or water holes. The smooth surface zone (hereinafter referred to as "undetectable zone") tends to regularly reflect the transmitted wave, and only a small fraction of the irregularly reflected wave is incident upon the receiver, whereby the optical energy received by the receiver is insufficient, and the output level of the receiver tends to be low. Therefore, when the vehicle is running on the undetectable zone, the output level of the receiver may be too low to detect the vehicle ground speed with high accuracy, leading to a risk that the receiving frequency of the wave as received by the receiver is lower than the actual frequency.
The known Doppler-effect vehicle ground speed detecting apparatus as disclosed in the above-identified U.S. Patents is designed to operate on the assumption that a change in the actual vehicle ground speed is the only cause for deviation of the receiving frequency from the optimum range. Consequently, the frequency changing means is activated to change the transmitting frequency, even when the receiving frequency deviates from the optimum range due to running of the vehicle on the undetectable zone.
Unlike a change of the transmitting frequency during running of the vehicle on an ordinary or normal ground surface, a change in the transmitting frequency during running of the vehicle on the undetectable zone does not significantly improve the sensitivity of the receiver. That is, a change in the transmitting frequency during running of the vehicle on the undetectable zone due to a change in the receiving frequency does not provide an appreciable effect on the receiver sensitivity, but rather has an adverse influence on the sensitivity soon after the vehicle has left the undetectable zone. Described more specifically, the transmitting frequency tends to be excessively increased with respect to the optimum range during the vehicle running on the undetectable zone, whereby the transmitting frequency may be too high to accurately detect the vehicle ground speed during an initial period following the passage of the vehicle through the undetectable zone. Since the apparatus in question is adapted to adjust the transmitting frequency so as to establish better detecting condition for accurate detection of the vehicle ground speed even on the undetectable zone, a result of this adjustment is an excessive increase in the transmitting frequency above the optimum level for the normal ground surface condition.